The monolithic integration of InGaN-based multi-color light-emitting diodes (LEDs) exerts a great impact on the full-color application field. The two dimensional arrangement of three primary colors micro-LED is expected to be used as a semiconductor video panel [1]. The emission color of InGaN/GaN triangular latticed nanocolumn arrays with the same lattice constant (L) is controlled by the nanocolumn diameter (D) [2]. For a blue light emission, the narrow nanocolumns should be utilized, resulting in a low filling factor of nanocolumns. However, the use of high filling factor of nanocolumn system is suitable for a stable device fabrication. In this study, therefore, we fabricated the densely-packed regularly arranged InGaN nanocolumns (D/L > 0.9). For the nanocolumn arrays with high filling factors, it was found that the emission color shifted from blue to red with increasing L from 80 to 350 nm. The emission color change is attributed to the different mechanism from Ref. [2], which is investigated to be clarified. Using the emission color change for the high filling factor nanocolumn system, four-color (red, green, blue, and yellow; RGBY) micro light emitting diodes (LEDs) were integrated in a 20×20 μm2 area (hereinafter called “unit”) and the units were two-dimensionally arrayed in a 16×16 square lattice in 400×400 mm2 area. These nanocolumn micro-LEDs were independently driven using matrix wiring electrodes, exhibiting RGBY light emissions.
[1] K. Kishino et al., Appl. Phys. Express 6, 012101 (2013). [2] H. Sekiguchi et al., Appl. Phys. Lett 96, 231104 (2008).
GaN nanocolumns are extensively studied as promising nano-materials for high-performance visible emitters because of their dislocation filtering and strain relaxation effects. The size and position of nanocolumns were precisely controlled using Ti-mask selective-area growth (SAG) by RF-MBE, fabricating uniform arrays of pn-junction InGaN/GaN nanocolumns. The periodic arrangement in the nanocolumn arrays led to nanocolumn photonic crystal (PhC) effect. It is however, necessary to integrate a wave-guiding scheme in the nanocolumn system to activate efficiently the PhCs. In the experiment, triangle-lattice GaN nanocolumn arrays with the lattice constant from 280 to 350 nm were grown, followed by the growth of InGaN/GaN superlattice buffer, MQW, and p-type GaN cladding layers. In the upper region of pn-junction nanocolumns from SL to p-GaN, the nanocolumn diameter increased and introduced the increase in the equivalent refractive index, which acts to confine the optical field there. Thus, the optical mode propagated laterally, interacting with the nanocolumn PhC. The diffraction at the photonic band edge resulted in high-directional beam radiations from the nanocolumn system. The photonic band edge was systematically investigated for various nanocolumn arrays with L=280–250 nm. The experimental photonic band diagram for the triangular-lattice pn-junction InGaN/GaN nanocolumn array exhibited a clear photonic band edge.
In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble,
Anderson localization of light is possible. We show localization parameter maps for the light localization adopting
parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar
semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light
frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated
temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed
the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies
continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the
column ensemble.
The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using
degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with
different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization
effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be
understood by considering the density of states and electron occupation number of the conduction band. It was also
revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate
conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.
The characteristics of intersubband transitions in III-nitride quantum wells are promising for detectors and all-optical
switches through a high intrinsic speed (~1 THz), and can also provide a high optical saturation power and a desired
small negative chirp parameter in electroabsorption modulators. The high LO-phonon energy allows to improve the
operating temperature of THz emitters. Recent achievements and prospects for intersubband III-nitride photonic devices,
mainly for λ=1.55 μm, are briefly reviewed. Further, means to enhance material quality by achieving crack-free growth
of GaN/AlN multiple-quantum-well (MQW) structures, and by employing intersubband transitions in multiple-quantum-disk
(MQD) structures incorporated into dislocation free GaN nanocolumns are discussed. We investigate the occurrence
of cracks in MBE-grown GaN/AlN MQWs on GaN MOVPE templates with respect to the buffer layer, the number of
QWs and the temperature reduction rate after growth. Intersubband absorption in GaN/AlN MQDs in the wavelength
range 1.38-1.72 μm is demonstrated in three samples grown on Si(111).
Self-assembled GaN nanocolumns were grown on sapphire and Si substrates by rf-plasma-assisted molecular-beam-epitaxy,
clarifying the growth condition. The nanocolumn crystal showed a highly efficient photoluminescence (PL)
emission at the room temperature, which intensity was 4 times stronger than that of a high-quality GaN substrate.
InGaN/GaN quantum-disk nanocolumn LEDs were fabricated on n-type (111) Si substrates. For a macroscopic emission
area of 500-&mgr;m-diameter, a broad electro-luminescence (EL) emission spectrum extending from the blue to the red
region was observed. Microscopic EL measurement was performed for a 3-&mgr;m-diameter detection area, demonstrating a
drastic spectral narrowing. In the microscopic EL spectrum, no blue shift of the emission wavelength was observed when
the injection current increased. This suggests that the carrier localization or/and the piezo-electric field is minimized in
nanocolumns. Selective growth of GaN nanocolumns was performed by use of patterned pre-deposited Al layers.
We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime τ. For negative delaytimes corresponding to τ < 0, coherent spectrum oscillations have been observed. At τ ~ 0, asymmetric dispersion has been observed. For positive delaytimes, τ > 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.
GaN nanocolumn (sometimes called as nanorod, nanowire, and nanopillar) is a columnar single crystalline GaN nano-crystal having small diameter of from tens to hundreds nanometers. In this paper, photoluminescence (PL) study of GaN nanocolumns and InGaN multiple quantum disks (MQDs) embedded in the GaN nanocolumns grown by RF-plasma assisted molecular beam epitaxy are described. The room temperature PL peak intensity of GaN nanocolumns was several hundred times stronger than that of conventional GaN film with dislocation density of 3~5x109 cm-2. Stimulated emission with very low threshold optical power density of 198~290 kW/cm2 was observed for the GaN nanocolumns. InGaN MQDs also showed intense PL emission with peak wavelength of from 436 (blue) to 614 nm (red).
GaN nanocolumn based light emitting diodes (LEDs) with InGaN MQD active layer were successfully fabricated on n-type (111) Si substrates. The emission color of the nanocolumn LED was varied from violet to red by changing the growth conditions of InGaN MQD active layer. The full width at half maximum (FWHM) of emission spectrum of the LED emitting around 500 nm taken from whole semi-transparent top electrode area (Φ=500 μm) was 73.5 nm but from small area (Φ=3 μm) was 37.0 nm. This result suggests that emission spectrum of a single InGaN quantum disk have narrower FWHM but large area LED that contains huge number of nanocolumn LEDs have wider FWHM.
We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AlN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra-subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.
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